FIELD OF THE INVENTION
The present invention relates to a process for the preparation of hydrocarbons by a catalytic reaction of a mixture of carbon monoxide and hydrogen with a catalyst.
It is known to prepare liquid hydrocarbons by converting a hydrocarbonaceous feed (for instance natural gas) into synthesis gas (a mixture comprising mainly hydrogen and carbon monoxide) and subsequently converting catalytically the synthesis gas into liquid and gaseous hydrocarbons. The catalytic reaction of the mixture of carbon monoxide and hydrogen at elevated temperature and pressure with a catalyst is known in literature as the Fischer-Tropsch hydrocarbon synthesis process.
These Fischer-Tropsch catalysts comprise one or more metals from the iron group together with one or more promotors deposited on a carrier material. More specifically such a catalyst comprises 3-80 parts by weight of cobalt, especially 15-50 parts by weight of cobalt, and 0.1-100 parts by weight of at least one other metal selected from the group consisting of zirconium, titanium, chromium and ruthenium, preferably 5-40 parts by weight of zirconium per 100 parts by weight carrier material. As carrier materials may be exemplified silica, zirconia, alumina or silica-alumina. Preferably silica is used. Suitable techniques for the preparation of these catalysts comprise precipitation, impregnation, kneading and melting. For further information reference is made to EP-A-127,220 and US 4,599,481.
These Fischer-Tropsch catalysts are very sensitive to impurities present in the mixture of carbon monoxide and hydrogen. These impurities, for instance NH.sub.3 and HCN, poison the catalyst, resulting in a decrease of the activity and specificity of the catalyst. These poisoning impurities are produced in harmful concentrations in various processes for the formation of the mixture of carbon monoxide and hydrogen, such as steam reforming or partial oxidation of light hydrocarbons, such as natural gas, particularly methane.
The poisoning impurities consist mainly of HCN and NH.sub.3. Sulphur containing compounds are usually already removed before the formation of the mixture of carbon monoxide and hydrogen, whereas NO.sub.x only affects the sensitivity of the Fischer-Tropsch catalyst to a minor extent. The removal of HCN and NH.sub.3 to concentrations below 1 ppm requires very large amounts of water. Water from a river or lake might be used, but requires a purification treatment. Salty sea water cannot be used.
It is an object of the invention to provide a process for the preparation of hydrocarbons from synthesis gas, comprising carbon monoxide and hydrogen, in which the use of extraneous water and the associated water purification costs are avoided. The invention is based on the insight that the catalytic formation of the hydrocarbons results in the formation of a sufficient amount of reaction water having a sufficient purity, so that this reaction water may be used for the removal of the poisoning impurities from the synthesis gas.